If mammals had arisen late and helped to drive dinosaurs to their doom, then we could legitimately propose a scenario of expected progress. But dinosaurs remained dominant and probably became extinct only as a quirky result of the most unpredictable of all events - a mass dying triggered by extraterrestrial impact. If dinosaurs had not died in this event, they would probably still dominate the domain of large-bodied vertebrates, as they had for so long with such conspicuous success, and mammals would still be small creatures in the interstices of their world. [...] Since dinosaurs were not moving toward markedly larger brains, and since such a prospect may lie outside the capabilities of reptilian design, we must assume that consciousness would not have evolved on our planet if a cosmic catastrophe had not claimed the dinosaurs as victims. (Gould 1989, 318)The context is the controversy around convergence and contingency in evolution. Rosenhouse discusses convergence as one of the hopes of Christians trying to reconcile evolution and Christian teachings, citing various proponents of the idea that their god set up the universe in a way that human-like intelligence was guaranteed to arise, thus producing beings that can have a "relationship" with said god.
Convergence is, of course, not only an observation considered helpful by the proponents of one variant of theistic evolution. To what degree the organisms that evolved on our planet would again turn out to be kind of similar if we replayed the tape or if organisms on other planets can be expected to look very similar to those on ours are very interesting questions of broad interest. Even an atheist may ask if we can expect lots of other planets where life arose to produce land plants, something a bit like insects, and perhaps even sentient beings given enough time, or if the vast majority of them will, for example, remain populated only by bacteria, because even evolving as much as multicellularity was a rare fluke.
Rosenhouse cites Gould as a well-known proponent of the importance of contingency. Although I tend much more towards the opposite view, I understand Gould's position. I believe the strongest argument for the contingency side is that while there are many impressive cases of convergence there are also quite a few crucial events in the history of life on this planet that appear to have happened only once: complex Eukaryotic cells; colonisation of dry land by multi-cellular plants; vertebrates; and of course human-like intelligence.
If, for example, the independent evolution of wings by insects, pterosaurs, birds and bats is counted as evidence for the importance of convergence, should something happening only once not be counted as evidence for the importance of contingency? My response would be competition, or in other words the change in the adaptive landscape caused by the first organisms to settle on a new peak. Where there may have been a ridge connecting the niches "kelp" and "large land-living plant" when nobody had occupied the latter, the first lineage to do so quickly became so good at being large land-living plants that the ridge crumbled away and became a canyon. If all land plants were wiped out, however, I would expect the land to be colonised anew, this time perhaps by red or brown algae.
But that is not actually about the main argument Gould is quoted as making in the above excerpt, and not what I found interesting about the quote. To take it in smaller pieces:
If mammals had arisen late and helped to drive dinosaurs to their doom, then we could legitimately propose a scenario of expected progress."Expected progress" is a bit of an odd term here. I am not sure if that is what is meant, but it could be read as if any group of animals that does not evolve towards large brains and intelligence is a refutation of the possibility that one group on each planet might evolve towards larger brains. But I do not think that this works as a refutation. And few proponents of the importance of convergence would argue that it is all about one linear progression towards large brains anyway. There are also progressions, for example towards body shapes that work well for swimming, towards paternal care for the young, towards powered flight, etc., and all of these happen at the same time but only in those lineages for which they solve relevant problems or create new opportunities.
If I understand the argument correctly, it is like pointing at a hole in the ground and saying, "if I now throw a pebble into the air and it does not end up in this specific hole, gravity is refuted", whereas the argument for convergence is that, what with evolution throwing thousands of pebbles into the air every year, we are very likely to find a few of them at the bottom of this hole as opposed to half way up its wall.
But dinosaurs remained dominant and probably became extinct only as a quirky result of the most unpredictable of all events - a mass dying triggered by extraterrestrial impact. If dinosaurs had not died in this event, they would probably still dominate the domain of large-bodied vertebrates, as they had for so long with such conspicuous success, and mammals would still be small creatures in the interstices of their world.Although this is not my field, and I understand that it is an active area of research, I believe it can already be said with some confidence that mass extinction is not random. There are generally some reasons for why an extinction event claims this lineage here but leaves that other one over there largely intact. If a mass extinction of marine life is caused, for example, by a massive drop in the oxygen content of the oceans, then we would expect lineages that can survive under low oxygen conditions to come out in relatively good shape, all things considered, while those with a high oxygen need would be hammered.
In the present case, if we hypothesise that the impact of a large meteorite would have caused massive shockwaves followed by a few years of something like nuclear winter, we could expect the following: Species of small animals may find it easier to survive because they need less food per number of individuals. Bonus points if you have a burrow to hide in when the devastation sweeps across your area (small mammals) or if you can move easily to other areas where a bit more food is left (flight-capable birds). Large animals that can go with little food for long times may also have a good chance, in other words being cold-blooded may help to survive several bad years (crocodiles). If, however, you are large and (!) at the same time you have a high rate of metabolism then you might be in trouble, as you constantly need lots of food per number of individuals. As far as I understand, that describes the non-avian dinosaurs: large and warm-blooded.
The point is, catastrophes do happen from time to time, and once one happened it would probably have decimated the largest animals, even if it had come ten million years later than it did. Their niches are filled up again by small animals evolving to be large (another good example of convergence). What killed off the pterosaur lineage, for example, may well have been that the birds had already out-competed all small pterosaurs, leaving only the very large species when the meteorite struck. But again, this is not my area of expertise really.
Since dinosaurs were not moving toward markedly larger brains, and since such a prospect may lie outside the capabilities of reptilian design, we must assume that consciousness would not have evolved on our planet if a cosmic catastrophe had not claimed the dinosaurs as victims.And this last part is really what I find the most interesting, because it illustrates so nicely how paraphyletic taxa can confuse the thinking even of the smartest of us, even of experts in evolutionary biology. What is the problem with the argument here?
First, and most obviously, birds are dinosaurs. Second, corvids (crows and ravens) and parrots are highly intelligent. Not quite human-level intelligence, but in some experiments corvids have proved to be smarter even than chimpanzees, our closest relatives. It follows that dinosaurs have actually "moved toward markedly larger brains", meaning here relative to the size of the body as a whole and, crucially, in terms of actual intelligence. Gould's premise is simply false, but his mistake is understandable, because at fault is really a misleading, i.e. non-phylogenetic, classification.
"Outside the capabilities of reptilian design" is, by the way, the same mistake at a deeper phylogenetic level. Mammals were not created fully formed, as mammals. Some of our ancestors were "reptiles", and here we are, having human-like intelligence by definition, what with us being humans and all that, so apparently there was a way of evolving human-like intelligence from a reptilian starting point. And from a fish starting point, and from a worm starting point, and from a bacterial starting point. All it took was lots of time and open niches waiting to be filled.
But I am not saying that anything here decisively refutes the idea that our sentience is a very rare fluke, unlikely to happen again should we go extinct. Maybe it is. The point is really how corrosive paraphyletic taxa are to reasoning about evolutionary processes.
Gould SJ, 1989. Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton, New York.